Thermal insulating materials which are used, for example, in the aerospace industries must meet some difficult requirements. The materials are often used at very high temperatures. The demands for low weight materials or weight reduction means that to increase insulation one cannot simply increase the number of layers or the thickness of the insulation. The insulation is used frequently to cover intricate shapes and so must be flexible enough to be shaped. Further, the materials are frequently subjected to severe environmental conditions, including high temperatures and moisture conditions.
One approach has been to make high temperature resistant fibers into flexible shapes such as "papers". These can provide dimensional requirements which allow for the shaping of the insulation while being light in weight. These sheet materials are known in the art as "papers" because they are often made by paper-making methods, although the thickness thereof can be up to one-half inch or more. These papers can be made by laying staple fibers into a mat and consolidating the mat into a paper, although other processes may be used. The staple fibers used in making these papers are heat resistant inorganic fibers such as glass, metal or ceramic fibers. By virtue of the laying operation, the staple fibers are randomly oriented and, with consolidation, are interlocked together into the form of a shape sustaining paper having two lateral surfaces. The papers are used as protective lining against high temperature, but they have other uses, such as a high temperature filtering medium.
Papers which have been used as insulation materials often have incorporated in them "filler" materials, such as aerogels of silica, chromic oxide, thoria, magnesium hydrate, alumina or mixtures thereof. The fillers, usually in the form of particles, serve to increase the density of the paper, lower its thermal conductivity and improve its insulation value. The filler particles are held by the reinforcing skeleton or network of staple reinforcing fibers, i.e., the paper, often aided by the use of organic or inorganic binders or by utilizing some inherent adhesive quality of the particles. This produces sheets of insulation material which have some flexibility and can be shaped and molded. Examples of these are given in U.S. Pat. Nos. 2,808,338, 2,811,457, 3,055,831 and 4,221,672.
To avoid the loss of structural integrity of the fiber skeleton during mechanical action, e.g. abrasion, flexing, and the like, the papers often have a binder applied thereto. These binders take various forms, but generally speaking the binders are organic polymers such as phenolics, acrylic and epoxies. The binders serve to improve the structural integrity of the papers during manufacture and fabrication of the papers into products.
However, the binders of these papers, while quite satisfactory at ambient temperatures, will begin to lose the binding effect at elevated temperatures, with a consequential loss of structural integrity of the papers. At even higher temperatures, and the temperatures at which these protective papers are normally used, the binder will burn away and the structural integrity of the paper and/or the filler particles will depend entirely upon the interlocking of the fibers. This is not as problem for papers which have been mechanically manipulated, conformed and fitted to the configuration of the particular apparatus in which it is used, it is most often held in place by the apparatus itself and substantial independent structural integrity of the paper itself is not required.
One solution to improve the mechanical life of such papers is that proposed in U.S. Pat. No. 4,499,134 to E. F. Whitely et al, the teachings of which are incorporated herein by reference. Whitely's solution was to mechanically bond a scrim to the outside of the paper by a network of threads. This "quilted" composite was then better able to retain the structural integrity of the paper in abrasive, high temperature environments.
But, the mechanical bonding is not adequate if the filler particles incorporated into the paper to increase its density and thermal resistance become unbonded or loose due to vibration or other mechanical or thermal action. Further, the filler particles may be adequate at high temperatures, but their performance or characteristics may be affected or changed under atmospheric conditions which cause the paper to become wetted and dried.
It would be, therefore, of considerable advantage to the art to provide improved papers, of the nature described above, wherein the structural integrity or insulating value of the papers can be largely maintained at higher temperatures if the binder burns away from the papers or where the papers are subject to wetting or to abrasion.
It is, therefore, an object of the present invention to provide such materials having improved structural integrity, improved insulation values and suitable for use in an abrasive, high temperature environment. It is a further object of the invention to provide such materials where the structural integrity and insulation value is largely maintained even when the material is used at temperatures sufficiently high that a binder material is burned away. It is a further object of the invention to provide such materials which can be made relatively inexpensively, are easily manipulated, shaped, formed for use both for original construction purposes and for repair purposes. It is a further object of the invention to provide methods for producing such materials. Other objects will be apparent from the following description of the invention and from the annexed claims.